1. Field of the Invention
The present invention relates to an optical wavelength converter, in particularly to a fiber type wavelength converter using Cerenkov radiation phase matching.
2. Description of the Related Art
Wavelength converters have been actively developed, which use nonlinear optical crystal to constitute an optical waveguide passage to guide an optical wave to a minute area, and effectively generate a second harmonic wave. These wavelength converters are generally classified into two types depending on the methods satisfying phase matching. The first type matches the phase velocity of a nonlinear polarization wave excited by an incident light of a fundamental wave with that of the second harmonic wave, and executes the phase matching between both the guide modes of the fundamental wave and of the second harmonic wave. The other type executes so-called Cerenkov radiation phase matching, i.e., the phase matching between the guide mode of the fundamental wave and the radiation mode of the second harmonic wave.
An optical wavelength converter is known, which is constituted in the shape of an optical fiber comprising a core made of nonlinear optical crystal and a clad surrounding the core. This optical wavelength converter employs the Cerenkov radiation phase matching. This optical wavelength converter is also known as an optical fiber type second harmonic wave generator (hereinafter referred to as "SHG").
FIG. 1 is a conceptual diagram of an SHG 3, which comprises a columnar core 10 and a cylindrical clad layer 20 concentrically surrounding the core 10. When the fundamental wave propagates through the core 10 from the left to the right in the diagram, a second harmonic wave is generated. In other words, the nonlinear polarization wave propagates at the same phase velocity and generates the second harmonic waves with a predetermined angle to the clad layer. The second harmonic waves are reflected within the inside surface of the clad layer 20 and propagate from the left to the right in the diagram. The phase matching between the guide mode of the fundamental wave and the radiation mode of the second harmonic wave is executed in the clad layer and the core.
The second harmonic wave and reflected wave at the boundary between the clad layer 20 are emitted from the end of the fiber in a corn shape as shown in FIG. 1. The equiphase wave surface of the wavefront of the thus emitted second harmonic wave is conical with the center axis of the fiber as its axis.
According to the Cerenkov radiation system, it is possible to generate the second harmonic wave whose optical phase is almost automatically matched. The SHG is therefore applied to a short-wave light generator. As shown in FIG. 3, the short-wave light generator comprises a semiconductor laser 1, a coupling lens 2, an SHG 3 of which core is constituted by nonlinear optical crystal, and an axicon lens 4. The coupling lens 2 collects and guides the light emitted from the semiconductor laser 1 onto the end face of the SHG 3. The axicon lens 4, shapes the wavefront of the second harmonic wave, irradiated after conversion done by the optical wavelength converter, to form the second harmonic wave in the form of the parallel flux of light.
In this way, the short-wave light generator module is constituted by the above SHG. However, the optical nonlinear material of this type is not found yet which has an efficiently large nonlinear polarization constant. In addition, it is difficult to select a material of claddings having a suitable refractive index for the core surrounded by the clad. Consequently, the wavelength converting efficiency (the output power of an second harmonic wave to be emitted/the output power of the semiconductor laser, i.e. The output power of the second harmonic wave) is about 0.1% at maximum.